Late Tertiary/Quaternary magnetostratigraphy and biostratigraphy of Vema Channel sediments

Abstract

Twenty-one piston cores in a transect of the Vema Channel near 31°S were analyzed magnetostratigraphically and biostratigraphically in order to ascertain the depositional continuity in the late Tertiary and Quaternary. The cores span a depth interval of 2941–4235 m and represent three distinct physiographic provinces, each of which shows a relatively consistent accumulation pattern. The deepest cores are from the broad terraces which flank the Vema Channel axis on both the west (∼3650 mm) and east (4150–4250 m); these cores show the highest accumulation rates in the region (>20 m m.y. −2) and no evidence of unconformities. Twelve cores from the flanks of the Rio Grande Rise (2941–3950 m) show depositional continuity in the late Quaternary and normal pelagic accumulation rates (5–10 m m.y. −2), with evidence in two of the cores for minor erosional hiatuses on a local scale, perhaps due to slumping. A relatively narrow transition zone between the Rio Grande Rise flanks and the eastern terrace (3950–4150 m) shows buried manganese pavements and near-outcrops of Miocene/Pliocene strata, indicating intermittent erosion during the late Tertiary and Quaternary, perhaps due to episodes of intensified AABW flow. Modern flow within this transition zone and over the eastern terrace is relatively weak ( \\ ̄ gu ≈ 2 cm s −2 ) and southward. The presence of well-developed sediment waves on the terrace and buried erosional unconformities upslope to the east suggests substantially higher current velocities during the late Tertiary than those present today.